Client close notification

There is **no** notification event for client close.
Is it possible to add AsyncListener#onClose() when the client is disconnected ? This will be
useful for long running connections like server-sent events connection.

On the other hand, SE socket API itself doesn't provide that option. So there may be no way to get client close notification when there is no pending i/o.

Start by reviewing the AsyncListener API and the Servlet specification sections covering client disconnects and asynchronous processing. Check how server-sent events connections and the SE socket API expose connection closure, then determine whether a reliable onClose notification can be specified. Done means a documented API design or a clear finding that the notification cannot be provided when no I/O is pending.

Written by the indexing model from the issue text.